This invention relates to the removal of SO.sub.x and NO.sub.x compounds from waste gas streams. More particularly, the invention relates to the removal of SO.sub.2, SO.sub.3, NO, and NO.sub.2 from industrial stack gases.
The burning of coal or oil as fuel in a boiler or furnace produces a flue gas (or stack gas) usually containing SO.sub.2 and SO.sub.3. The concentration of these sulfur compounds in a particular stack gas depends upon such factors as the concentration of sulfur components in the fuel, the metals concentration in the fuel, and the air rate fed to the boiler or furnace. A stack gas, however, will usually contain between about 100 ppmv and 5 mole percent SO.sub.x compounds, with the large majority thereof, usually at least about 95% thereof, being in the form of SO.sub.2.
Before a stack gas containing SO.sub.x compounds may be discharged to the atmosphere, many environmental regulatory agencies require that the stack gas be desulfurized, that is, that the concentration of sulfur compounds therein be reduced to specified levels. In California, for example, gases containing more than 500 ppmv of total sulfur compounds (calculated as SO.sub.2) may not be discharged to the atmosphere. Hence, stack gases containing more than 500 ppmv sulfur compounds must be desulfurized prior to discharge to the atmosphere.
Two of the most conventional methods presently being utilized to remove SO.sub.x compounds from stack gases involve contacting the stack gas with lime or caustic. Such processes, however, while favorable from a cost standpoint, produce a relatively large amount of calcium sulfate, sodium sulfate or other undesirable by-products that represent a difficult waste disposal problem. Accordingly, it is an object of this invention to provide a process for desulfurizing stack gases while at the same time reducing or eliminating the problem of solid waste disposal. It is another object of this invention to provide a process wherein sulfur compounds removed from the stack gas are dissolved in an aqueous absorbent medium, which aqueous absorbent medium is subsequently regenerated by reducing the dissolved sulfur components to elemental sulfur under mild conditions of temperature and pressure. In addition, because many stack gases contain NO.sub.x compounds as well as SO.sub.x compounds, said NO.sub.x compounds being formed during combustion at least in part by the reaction of nitrogen and oxygen in the air supporting combustion, it is a further object of the invention to provide an aqueous adsorbent medium for removing both SO.sub.x and NO.sub.x pollutants, which absorbent is capable of being regenerated in a single regeneration vessel to a form active for removing SO.sub.x and NO.sub.x compounds.